You may find retired sets from various themes on sites like Bricklink, but the old Batman sets in particular were retired fairly quickly compared to other sets, so the demand is high. In other words, if you're looking for MISB or even open/complete sets, it's highly unlikely that you'd get them at the original price.

The reason they are so expensive is because they aren't being made any more so they aren't available at retail, there aren't that many copies available for sale from secondary sources, and plenty of people want them. Under those circumstances, those that DO have copies they are willing to sell can ask high prices for them, knowing that SOMEBODY is likely willing to pay them.

In short, pretty much all LEGO sets appreciate in value after they go out of production. As far as Bricklink goes, you'd need to register to purchase anything...you can think of Bricklink as eBay specifically for LEGO and with everything as "Buy it now." It's a conglomerate of sellers centralized on one site.

If you are trying to get something before Christmas at that price then good luck to you. You won't find any "black Friday" deals on bricklink or ebay especially that close to the holidays. On Bricklink you can check the selling history to see what sets have been selling for recently to get an idea. You can look at ebay but be prepared to spend time doing this periodically checking because you will find people that are just selling old sets who aren't resellers but again it does take time. I have bought several old Star Wars sets this way for fairly cheap.